Recently, a cooler and refrigerator for professional-use, for example, provided with an inverter compressor capable of speed control has been used (See Patent Document 1, for example).
There are several advantages to provide an inverter compressor and one of them is higher efficiency in controlled cooling operation. This control is provided to lower a speed (number of rotations) of the inverter compressor in a stepped manner in the vicinity of a set temperature during the controlled cooling operation for maintaining the inside of the cooler and refrigerator in the vicinity of the set temperature. With this control method, a continuous ON time of the compressor becomes overwhelmingly longer, in other words, the number of switching times between ON and OFF is drastically reduced and operation is carried out at a low rotation, which promotes higher efficiency and saving of energy.
In the above control, cooling capacity during low-speed operation of the inverter compressor needs to be set to exceed an assumed standard thermal load. If the cooling capacity does not reach the assumed thermal load, the inside temperature does not fall to the set temperature but is thermally balanced and does not reach the set temperature.
[Patent Document 1]: Japanese Patent Laid-Open No. 2002-195719
Though the inverter compressor is operated at a low speed, the thermal load can get larger than expected in some cases, particularly with a cooler and refrigerator for professional-use depending on conditions such as heat capacity of food materials, ambient temperature, and opening/closing frequency of a door. Also, there is a possibility that the inside temperature remains before the set temperature for a long time or even though the temperature drops, the change is too slight, resulting in the ON time becoming abnormally long.
However, continuous operation while the compressor is kept ON in this way is not particularly preferable. That is because frost keeps on adhering to a cooler by air intruding from outside at opening/closing of the door or steam from the food materials, and its gradual growth results in lower heat exchanging capability of the cooler.
The inventors have already developed a technology to prevent continuous cooling operation by carrying out operation that the inside is cooled all the time at a cooling speed determined in advance in the controlled cooling operation (See Patent Application No. 2003-359715, for example). This technology is configured so that data indicating a preferable temporal change mode of temperature drop (target cooling speed) is stored in advance, actual cooling speed is measured at the controlled cooling operation and compared with the data, and the cooling operation is carried out so that the both the target cooling speed and the controlled cooling operation accord to each other.